1. Field of the Invention
Embodiments of the invention are directed to a formulation and method for making plasticized fluff pulp that is partially de-bonded, soft, and resilient. Other embodiments are directed to absorbent materials made from such plasticized fluff pulp to be used as an absorbent core in absorbent articles such as disposable diapers, feminine hygiene products, and incontinence devices. More specifically, embodiments of the invention are directed to a plasticized fluff pulp that is useful in making absorbent core material that is soft, flexible, resilient and free of hard spots.
2. Description of Related Art
Products containing absorbent fluff pulps are used in a wide variety of personal care products. These range from absorbent articles such as personal hygiene products to wipes or pads used in medical and food handling applications. Incorporation of superabsorbent materials into absorbent articles intended for personal hygiene products has led to a noteworthy reduction in the use of fluff pulp, but the continued growth of feminine hygiene and incontinence device markets has ensured that there is a continued need for absorbent fluff pulp.
While the design of individual absorbent articles varies depending upon intended use, there are certain elements or components common to such articles. Absorbent articles intended for personal care, such as adult incontinent pads, feminine care products, and infant diapers typically are comprised of at least a top sheet, a back sheet, an absorbent core positioned between the top sheet and back sheet, and an optional acquisition/distribution layer positioned between the top sheet and the absorbent core. The function of the absorbent core is to absorb and store body fluids entering the absorbent article through the top sheet layer. Because the origin of body fluids is localized, it is desirable to provide a means for distributing fluid throughout the dimensions of the absorbent core to optimize the use of the available absorbent material. This is typically accomplished either by providing an acquisition/distribution member positioned between the top sheet and absorbent core and/or altering the composition of the absorbent core per se.
The acquisition/distribution layer typically is incorporated in the absorbent articles to provide better distribution of liquid, increased rate of liquid absorption, reduced gel blocking, and improved surface dryness. Acquisition/distribution layers usually are comprised of, for example, acquisition fibers or other material that retains small amounts of fluid. A wide variety of acquisition fibers are known in the art. Included among these are synthetic fibers, a composite of cellulosic fibers and synthetic fibers, and cross-linked cellulosic fibers.
The absorbent core is typically comprised of a wood fluff pulp that is capable of absorbing large quantities of fluid and retaining a small amount of fluid. Absorbent cores can be designed in a variety of ways to enhance fluid absorption and retention properties. By way of example, the fluid retention characteristics of absorbent cores can be greatly enhanced by distributing superabsorbent materials among the wood fluff pulp fibers. Superabsorbent materials or polymers (SAP) are well known in the art as substantially water-insoluble, water-swellable materials capable of absorbing water in quantities up to 100 times their weight or more. Absorbent cores for hygienic products, particularly diapers and adult incontinence products, are usually manufactured on a continuous production line in which wood fluff pulp is provided in roll form (usually manufactured by wet-laid process) and is defiberized by mechanical means, such as a hammermill. The defiberized fluff pulp then is conveyed to a forming area where it is air-laid with particles of SAP, in a predetermined ratio, to form an air-laid absorbent core. The air-laid absorbent core is either inserted into an absorbent article, or wound on a roll for later introduction to the manufacturing of an absorbent article.
Typically, absorbent articles are designed to be absorbent, thin and flexible. In recent years, as consumer demand for less expensive and less bulky disposable absorbent products has increased, manufacturers have sought effective ways to reduce the size and cost of the products without sacrificing the fluid transport properties or structural integrity of the products during use. Notably, absorbent articles have become progressively thinner over the last decade. For example, the thickness of a feminine hygiene pad has been reduced from about 15 mm to 20 mm in the mid 1980's to about 2.5 mm to 6 mm today. Unfortunately, the shift to thin and ultra thin products has not been without manufacturing problems. For instance, as the products became thinner, absorbent cores lost integrity. To counter this, absorbent article designers have tried to produce higher integrity cores, such as by compressing the core to a high density and/or use bonding agents to achieve fiber-to-fiber and fiber-to-SAP particle bonding. However, the increased density and increased usage of SAP in these products has caused problems with liquid acquisition and wicking rates. Moreover, compressing the absorbent core to a high density can cause the core to develop hard spots (clusters of SAP and fibers with very high density) that are undesirable to consumers.
In an attempt to overcome these problems, debonding agents such as those disclosed in U.S. Pat. Nos. 3,554,862; 3,677,886; 3,809,604; 4,144,122 and 4,432,833 have been used. Debonding agents usually are quaternary ammonium compounds containing one or more fatty groups that soften and lubricate the fibers. When applied to a sheet of wood pulp fibers, the fatty groups disrupt the inter-fiber hydrogen bonding (fiber-to-fiber bonding)—as a result, voids are created among the fibers. These voids enhance the bulk of the fibers, thereby producing a softer and weaker sheet of wood pulp. Similarly, cationic materials such as those disclosed in U.S. Pat. Nos. 3,554,862; 3,677,886; 3,809,604; 4,144,122 and 4,432,833 and nonionic agents such as BEROCELL 587 (available from Eka Chemicals, Inc.) also have been used on wood pulp. The use of non-ionic agents, such as fatty acid esters, in combination with cationic retention agents has been disclosed, for example, in U.S. Pat. No. 4,303,471, and is known to produce good disintegration properties for wood pulp. Unfortunately, the long hydrophobic alkane chains in these softening and debonding agents tend to have undesirable hydrophobic effects on pulps. For example, they tend to decrease the absorbency and wettability of the pulp, thereby rendering the pulp unsuitable for applications such as absorbent articles, where high absorbency and fast wicking are desirable. Moreover, the softened and de-bonded fluff pulps tend to form more hard spots than conventional untreated fluff pulp when calendered with SAP particles.
Another proposed solution for improving softness of densified absorbent cores is to use mercerized fibers. The use of mercerized fibers to enhance the softness of the absorbent cores has been disclosed in U.S. Pat. No. 5,866,242. However these fibers are expensive when compared to non-mercerized fibers.
As an alternative to the use of additives or mercerized fibers, plasticizing agents such as those disclosed in U.S. Pat. Nos. 4,098,996; 5,547,541; and 4,731,269 also have been used as a softener for wood pulp. Typically, a plasticizing agent is added to a pulp slurry prior to forming wet-laid sheets. The plasticizing agent is added in large quantities of at least 10 weight % of pulp. The resulting pulp sheet usually lacks stiffness, and is easy to densify when air-laid to a nonwoven pad. Common plasticizing agents include polyhydroxy compounds such as glycerol; low molecular weight polyglycols such as polyethylene glycols and polypropylene glycols; and other polyhydroxy compounds. Ammonia, urea, and alkylamines are also known to plasticize wood pulp (see A. J. Stamm, FOREST PRODUCTS JOURNAL 5(6):413, 1955). One draw-back to these plasticizers is that they need to be added to the pulp slurry in large quantities, which has an adverse effect on the wet-strength of the absorbent core. Moreover, in order to provide beneficial properties to pulp these additives have to be added to the pulp during the wet-laying process.
The description herein of certain advantages and disadvantages of known cellulosic fibers, treatment compositions, and methods of their preparation, is not intended to limit the scope of the present invention. Indeed, the present invention may include some or all of the methods, fibers and compositions described above without suffering from the same disadvantages.